Multicarrier signals generally have a high ratio between the signal maximum and the standard deviation of the signal. This ratio is also referred to as the crest factor and places stringent requirements on amplifiers and transmitters in order not to risk saturation effects which could result in loss of data. It is therefore actually necessary to reduce this crest factor for DMT (discrete multitone) and for OFDM (orthogonal frequency division multiplex) signals, in order to prevent saturation of the amplifier and transmitter and, furthermore, to reduce the power consumption of the amplifier and of the transmitter during transmission. If the crest factor is squared, then this results in the so-called PAR (peak-to-average ratio), which should likewise be reduced, for the reason mentioned above.
German Laid-Open Specification DE 198 50 642 A1 describes a method for reducing the crest factor of a signal. In this case, a signal is transformed using an IFT device and both a signal maximum and a signal minimum of the output signal are determined, from which a correction variable is derived. The output signal from the IFT device is corrected by means of the correction variable, which is derived from the determined value of the signal maximum and of the signal minimum, and with a second correction value possibly being calculated for correction of the crest factor of a signal. However, this has the disadvantage that any influence on the crest factor from downstream devices (amplifiers, converters, transducers, transformers, filters, etc.) are ignored in the correction process.
FIG. 4 shows a simplified block diagram of a number of schematic elements of a DMT or OFDM transmission device. A datastream 10 is subjected to inverse Fourier transformation in an IFT device 11. The multicarrier signal 12 is then, for example, passed to a high-pass filter 13, where it is filtered. The filtered output signal 14 is then supplied to an interpolation stage 15 and/or to an interpolation device with a low-pass filter. The filtered and interpolated output signal 16 is then converted in a block 17 to an analog signal, and is then filtered using a low-pass filter, before the output signal 18 from this converter device 17 with a low-pass filter is passed to an amplifier device (not shown).
DMT and OFDM signals are subject to the disadvantage that the ratio of the maximum to the standard deviation (crest factor) of the signal is very high. In order to reduce the requirements for a downstream output amplifier, particularly with regard to the linearity and the power consumption of the amplifier device, and for digital filters, as regards resolution, and for D/A converters, various methods are known from the literature which allow the crest factor to be reduced. The subject matter of most of the methods is to reduce the crest factor directly after the IFT device 11, for example starting at nodes 12′. However, this method is subject to the problem that the crest factor will rise again as a result of the downstream filters 13 and the interpolation with low-pass filtering in the block 15. However, in order to make it possible to reduce the power consumption of the downstream amplifier, it is necessary to reduce the output crest factor of the signal 18.
A more successful reduction to the crest factor can be achieved if the reduction is carried out after the interpolation in the block 15, that is to say starting for example at the node 16′. In the paper “PAR reduction revisited: an extension to Tellado's method”, which was published in conjunction with the sixth International OFDM Workshop (InOWo) 2001 in Hamburg, Werner Henkel and Valentin Zrno propose an advantageous method such as this which is explained, for example, in the published paper “Further Results on Peak-to-Average Ratio Reduction” by Jose Tellado and John M. Cioffi. According to the article by Henkel, the maximum value of the signal 16 is in this case determined after interpolation in the filter device 15, for each data frame of the input signal 10. This information, that is to say the precise sample value of the maximum value of the signal (both on the time axis or in the x direction as well as the amplitude of the maximum value, that is to say relating to the y direction) , is used in order to correct the output signal 12 from the transformation device 11, for example starting at the point 12′.
The corrected signal is then once again passed through the high-pass filter 13 and the interpolation device 15 and, if necessary, the described steps are repeated. This implementation according to Henkel and based on Tellado is subject to the disadvantage that all of the filters from the high-pass filter 13 and the interpolation device 15 must be taken into account for each iteration or repetition. This leads to time-consuming computation operations and thus to restricted practical usefulness of the Henkel method according to the prior art.